By Andrea Shalal RAF FAIRFORD England (Reuters) - The U.S. Navy on Friday maintained a grounding order for F-35 B-model and C-model fighter jets built by Lockheed Martin Corp, saying it was still not clear what caused a massive engine failure on an Air Force F-35 jet last month. "At this time, I do not have sufficient information to return the F-35B and F-35C fleet to flight," Vice Admiral David Dunaway, who heads the Navy's Air Systems Command, said in an update to a fleetwide grounding order issued by U.S. officials on July 3. Dunaway said in the document that he was committed to returning the F-35 fleet to flight as soon as possible, but there was "no discernible event that represents a root cause." In the incident on June 23, the Pratt & Whitney engine on an Air Force F-35 A-model jet broke apart and caught fire while a pilot was preparing to take off from a Florida air base. Until the grounding is lifted, the U.S. Marine Corps and Britain will not be able to ferry four F-35B aircraft to Britain for the jet's planned international debut at two air shows there this month - the Royal International Air Tattoo, the world's largest military air show that began Friday, and the Farnborough air show, which starts on Monday and runs until July 20.